1. Field of the Invention
The present invention relates to thermo optic cell image projection systems. More specifically, the present invention relates to applying a uniform variable electric field across the thermo optic cell to produce a uniform variable darkening of the thermo optic cell.
2. Summary of the Prior Art
Liquid crystal cells are useful in display, optical processing, printing and related image projection applications. In these applications they serve as light valves or light modulators which control the flow of light from a light source to a receiver or receiving surface. When suitable spatial electronic addressing means is incorporated in a liquid crystal light valve system, spatially varying patterns can be written on the liquid crystal layer. The liquid crystal cell then becomes a spatial light modulator. When suitable illumination optics, typically including a light source and optical condenser, and suitable projecting optics, typically including a projection lens with appropriate aperture, are included in a liquid crystal light valve system, the liquid crystal cell then becomes an electronic slide which determines and controls the image projected onto a display screen, photosensitive detector array or photosensitive writing medium for display, optical processing, and printing applications, respectively.
The spatial variations of the image on the liquid crystal cell virtually always correspond to differences in the molecular orientation (different textures) of the liquid crystal in different areal regions of the liquid crystal layer. The spatially varying differences in molecular orientation are converted into spatially varying light intensity variations by means of suitable polarizing optics or suitable apertures to selectively pass and block scattered or refracted light from these spatially different regions. The various possibilities have been reviewed in detail (see F. J. Kahn, "The Molecular Physics of Liquid Crystal Devices," Physics Today, May 1982).
In one mode of operation the entire image area of the liquid crystal cell is switched, typically by electrical or thermal means, into a texture with uniform order (or disorder) thereby providing a uniform image background. One or more spatially varying images with different textures are then superposed, by the spatial addressing means, onto the uniform image background in order to create an image on the liquid crystal cell, and hence on the receiving surface, with the desired spatial variations in intensity and contrast.
Referring to FIG. 1, cross sectional view of a typical prior art liquid crystal cell 10 is shown. The cell 10 is filled with liquid crystal 20. A suitable liquid crystal is smectic A liquid crystal. The crystal is contained on two sides by a first and second electrode 16 and 18. The electrodes 16 and 18 are substantially parallel plates made of transparent material. A suitable material for the electrodes is indium-tin-oxide. The electrodes 16 and 18 serve to allow application of an electric field in a direction essentially normal to the liquid crystal layer while allowing projection light to be transmitted through the liquid crystal layer. Adjacent the electrodes 16 and 18 are alignment layers 17 and 19 that serve as dielectric layers for electrical blocking and/or liquid crystal molecular alignment. The liquid crystal cell 10 is further comprised of a first and second plane of glass 12 and 14. Among other things, the glass permits the passage of light, supports the electrodes, and provides integrity to the liquid crystal cell.
In the image projection system context, liquid crystal cell image projection systems incorporate means for creating an image on a liquid crystal cell and then use the created images as a mask for projection exposure of photosensitive media for photolithographic processing such as fabrication of printed wiring boards. One of the initial steps in producing an image on a liquid crystal cell involves forming a uniform texture of scattering centers. Scattering centers refer to the characteristic of the liquid crystal material in the cell 10. Scattering centers scatter and depolarize light. Conversely, liquid crystal regions without scattering centers transmit light without deflecting it or depolarizing it. A uniform texture of scattering centers is a microscopic volume of liquid crystal cell 10 which scatters incident light.
The uniform texture of scattering centers is then "drawn" on by a laser beam to create an image. The laser beam forms an image in the uniform texture of scattering centers by creating non-scattering regions wherever it is impinged. The process of impinging laser light on the uniform texture of scattering centers to produce non-scattering centers is known in the art, (F. J. Kahn, "Locally Erasable Thermo-Optic Smectic Liquid Crystal Storage Displays," U.S. Pat. No. 3,796,999 issued Mar. 12, 1974).
Following creation of an image either by producing scattering centers within a non-scattering region, or by creating non-scattering regions within a uniformly darkened background, it is necessary to erase or refresh the liquid crystal cell in order to prepare it for generation of subsequent images.
The prior art contains two methods of rapidly producing large area uniform scattering textures by methods other than laser-scanning which is relatively slow. Both of these methods involve the use of electrical energy and as will be discussed below, are not capable of producing the uniform variable darkening required in high resolution applications. In the first method, an electrical current pulse through the electrode is used to generate heat in the liquid crystal, the electrode heating method. In the second method, an electrical current pulse through the liquid crystal is used to create a turbulent material flow in the liquid crystal, the current-turbulence method. In the first method (see File A-44550, allowed July 1, 1988) a signal from a voltage source 22 is applied across one of the electrodes. This method is illustrated in FIG. 1 where the voltage signal from the source 22 is applied across the electrode 18. In this embodiment, short electrical pulses are used to thermally induce a uniform texture of scattering centers over the substantially large surface area of the cell 10. The result, in an image projection system, is that the short electrical pulses result in a fast darkening of the receiving surface. The electrical current sent through the electrode 18 produces thermal energy which raises the temperature of the liquid crystal material 20. Also, because it is a short pulse, the energy does not have time to spread. This lack of spreading facilitates rapid cooling which is responsible for the creation of uniform scattering centers. Physically, the liquid crystal is heated to a temperature at which it is an isotropic state. It then cools rapidly, quenching in some of the disorder of the isotropic state and thereby produces a high density of scattering centers.
In an alternate embodiment of the prior art, the electrical pulses are applied across the liquid crystal 20. In this embodiment, following the work of Tani (C. Tani, Applied Physics Letters 19, 241-2, Oct. 1, 1973. "Novel Electro-Optical Storage Effect in a Certain Smectic Liquid Crystal"), a dc electric field applied across a thin layer of smectic A liquid crystal causes a current to flow in the liquid crystal. The current produces a turbulent flow of the liquid crystal analogous to the well-known dynamic scattering effect in nematic liquid crystals. This turbulence can produce a high density of optical scattering centers in smectic liquid crystal cells with appropriate conductivity and appropriate orienting layers on the substrates, and can also be produced by low frequency ac voltages as well a dc voltages. See for example, W. A. Crossland and S. Canter, "An Electrically Addressed Smectic Storage Device," SID '85 Digest, pp. 124-126 (1985) and W. A. Crossland, J. H. Morrissy, and B. Needham, "Method for Preparing and Operating a Smectic Liquid Crystal Display Cell Having Indefinite Storage Properties, U.S. Pat. No. 4,139,273 (Feb. 13, 1979).
There are two significant problems, however, with these two methods of creating and erasing uniform texture of scattering centers. Generally speaking, the first of these relates to producing images of high resolution. The second relates to degradation of the liquid crystal cell caused by electrical stresses.
Addressing the first problem, the generation of uniform texture of scattering centers produces a relatively constant magnitude of scattering. Basically, the liquid crystal material is heated to a high temperature by the electric pulse. As the cell cools the disorder created by the electrically induced thermal pulse is quenched into the cell. This level of disorder is responsible for uniform texture of scattering centers, but it produces a constant level of darkening. That constant level of darkening is too dark for high resolution application or too light for adequate contrast with the created image.
In high resolution applications it is desirable to have a lighter background, i.e., a background with less scattering effect. This allows for thinner lines to be drawn on the liquid crystal cell. Given the dark background of the electrically created cell, a thin line drawn across the screen is obscured by the closely and densely surrounding scattered centers. Thus, it is desirable to efficiently control the magnitude (or density) level of the cell in the darkening process.
Control of the level of darkening may be accomplished by reducing or increasing the amount of energy (electrical or optical) required to obtain the desired level of darkening. However, the scattering centers produced by any level of input energy have been shown (see F. J. Kahn et al, "A Paperless Plotter Display System Using a Laser Smectic Liquid-Crystal Light Valve," SID 87 Digest, pp. 254-257), to lie in a layer immediately adjacent to the surface where the conversion of optical or electrical energy into the terminal pulse takes place. The thickness of this layer is proportional to the level of input energy. That is, low energy produces a thin layer with few scattering centers (low optical density) and high energy produces a thick layer with many scattering centers (high optical density).
Since the generation of scattering centers is subject to many variables including the resistivity of electrodes, concentration of additives and dopants and local environmental variations, it is not practical to generate thin layers with the good uniformity of darkening required for fine lines and high resolution. The preferred operating mode would be to apply energy sufficient to allow darkening of the cell to an initial level considered to be a maximum required for any image. Then by imposing the electric field reduce the optical density as required to produce thin, high resolution lines with the highest possible contrast.
In the current-turbulence method, the liquid crystal typically must have conducting additives or dopants in it, in order to allow enough current flow to create the scattering center inducing turbulence. The passage of current through the liquid crystal, however, typically results in electrochemical processes which cause the devices to fail. Thus other dopants, such as redox dopants, must be added to the liquid crystal to counteract these electrochemical processes and extend lifetime.
Additionally, processes must be developed to distribute these dopants uniformly through the liquid crystal cell and special low resistance contacts or busbars (requiring expensive and yield reducing patterning steps in the fabrication process). Electrodes must be provided in order to minimize voltage drops that would result in nonuniform current distribution and undesirable heating. Thus, implementing either of these methods results in considerable additional expense in fabrication of the liquid crystal cells.
Finally, to erase the previously applied background darkening together with the overlaying image requires application of an electric field. Erasure of scattering centers by application of an electric field during slow cooling of the liquid crystal is well known in the art. However, this process is too slow for practical use. Erasure of scattering centers by scanning with a laser is described by Fredric J. Kahn, in U.S. Pat. No. 3,796,999 issued Mar. 12, 1974. This is a pointwise erasure process and is also slow.
Fast erasure of a darkened background together with the overlaying image is described in reference Fredric J. Kahn et al, "A Paperless Plotter Display System Using a Laser Smectic Liquid-Crystal Light Valve," SID 87 Digest, pp. 254-257. The technique uses very high electrical fields ranging from 10-15V/micrometer. This high field causes electrical and electro-chemical stress which reduces the life of the liquid crystal cell.